Shielding method and apparatus using flexible strip

ABSTRACT

A shielding method and apparatus for an antenna disposed on an elongated support adapted for disposal within a borehole. The shield providing predetermined attenuation of one or more electromagnetic energy field components as the field components interact with the shield. The shield composed of a flexible strip or cylindrical body and respectively comprising a transverse conductive element or a transverse slot therein. The shields being adapted to cover an antenna mounted on the support. The shields being compatible for use in conjunction with saddle, tilted coils or multi-layered tri-axial coils to produce a pure transverse magnetic dipole electromagnetic field. The shields are also used in methods for shielding an antenna disposed on a support to provide predetermined attenuation of an electromagnetic field component as the field components interact with the shield.

CROSS-REFERENCES

This application is a divisional of U.S. patent application Ser. No.09/746,927, filed Dec. 22, 2000 now U.S. Pat. No. 6,566,881, which is acontinuation-in-part of U.S. patent application Ser. No. 09/452,660,filed Dec. 1, 1999 entitled “Shield Apparatus for Use in Conjunctionwith a Well Tool, now U.S. Pat. No. 6,351,127.

1. BACKGROUND OF THE INVENTION

1.1 Field of the Invention

This invention relates to the field of well logging, and moreparticularly, to improved shields for use with logging instruments usingsources or sensors having a transverse or tilted magnetic dipole.

1.2 Description of Related Art

Electromagnetic (EM) logging instruments have been employed in the fieldof hydrocarbon exploration and production for many years. These logginginstruments or “sondes” entail an elongated support member equipped withantennas that are operable as sources and/or sensors. These antennas aregenerally coils of the cylindrical solenoid type and are comprised ofone or more turns of insulated conductor wire that is wound around thesupport. U.S. Pat. No. 4,873,488 (assigned to the present assignee), forexample, describes logging instruments equipped with antennas disposedalong a central support.

In operation, a transmitter antenna is energized by an alternatingcurrent to emit EM energy into the formation. The emitted energyinteracts with the surrounding formation to produce signals that aredetected and measured by one or more antennas. The measured signals arethen processed to determine the electrical properties, such aspermittivity or conductivity, of the formation.

Conventional EM logging techniques include “wireline” logging andlogging-while-drilling (LWD). Wireline logging entails lowering theinstrument into the borehole at the end of an electrical cable to obtainthe subsurface measurements as the instrument is moved along theborehole. LWD entails attaching the instrument disposed in a drillcollar to a drilling assembly while a borehole is being drilled throughearth formations. A new method sometimes referred to aslogging-while-tripping (LWT) involves placing a logging tool near thebottom of the drill string and making measurements while the string iswithdrawn from the borehole.

A coil carrying a current can be represented as a magnetic dipole havinga magnetic moment proportional to the product of the current and thearea encompassed by the coil. The direction and strength of the magneticmoment can be represented by a vector perpendicular to the plane of thecoil. In the case of more complicated coils which do not lie in a singleplane (i.e. saddle coils referenced later), the direction of the dipolemoment is given by: ∫r×dl and is perpendicular to the effective area ofthe coil. This integral relates to the standard definition of a magneticdipole of a circuit. See J. A. Stratton, Electromagnetic Theory, McGrawHill, New York, 1941, p. 235, FIG. 41. Integration is over the contourthat defines the coil, r is the position vector and dl is thedifferential segment of the contour.

In conventional induction and propagation logging instruments, thetransmitter and receiver antennas are mounted with their axes along, orparallel, to the longitudinal axis of the instrument. Thus, theseinstruments are implemented with antennas having longitudinal magneticdipoles (LMD).

If the transmitter and receiver antennas on these instruments wereperfectly configured and balanced in a theoretically ideal system, theEM energy emitted by the antennas would propagate in a mode known as atransverse electric (TE) mode, of the type generated by an idealvertical magnetic dipole in an azimuthally symmetric media. However,under actual operating conditions, there are various factors that giverise to the generation of significant undesired EM field components. Oneapproach to alleviating this problem is with the use of antenna shieldsto reduce the transmission and/or reception of spurious and unwanted EMfield components. These shields are typically used in conjunction witheach antenna on the instrument although they can be used with only someof the antennas. For instance, if each shield provides N dB attenuationof undesired modes, then having shields on both transmitters andreceivers will provide 2N dB of attenuation. If N dB is enough for themeasurements desired, then shields may be used only for the transmittersor only for the receivers.

U.S. Pat. Nos. 5,631,563, 4,808,929, 4,949,045, and 4,536,714 (allassigned to the present assignee) disclose conventional antenna shieldsused with these instruments to provide mechanical protection for theantennas and to permit the passage of desired EM field components. Asshown in FIG. 1a, these shields 10 are in the form of a metal cylinderthat has slots 12 in the axial direction. The slot 12 pattern allows theazimuthal electric field (Eφ) component to pass through with littleattenuation, while the radial (Er) and axial (Ez) are attenuated more asthey pass through the shield.

An alternative viewpoint is to represent each axial slot 12 as an axialmagnetic dipole, as shown in FIG. 1b. These magnetic dipoles couple toaxial-magnetic fields (Bz), but do not couple to azimuthal magnetic (Bφ)fields. The shielded antennas are thus rendered substantiallyinsensitive to parasitic transverse magnetic (TM) EM fields associatedwith borehole modes, and which have radial (Er) and axial (Ez) electricfields and azimuthal magnetic fields (Bφ).

An emerging technique in the field of well logging is the use ofinstruments with tilted or transverse antennas, i.e., where the coil'saxis is not parallel to the support axis. These instruments are thusimplemented with antennas having a transverse or tilted magnetic dipolemoment (TMD). One instrument configuration comprises tri-axial coils,involving three coils with magnetic moments that are not co-planar. Theaim of these TMD configurations is to provide EM measurements withdirected sensitivity and sensitivity to the anisotropic resistivityproperties of the formation.

Logging instruments equipped with TMDs are described in U.S. Pat. Nos.6,044,325, 4,319,191, 5,115,198, 5,508,616, 5,757,191, 5,781,436 and6,147,496. Common to these apparatus and techniques, however, is theneed to manipulate the antenna itself. None of these disclosures addressthe implementation of shields as alternative means to achieve selectiveEM energy attenuation.

A transverse slot concept has been used in design of high frequencycommunication antennas. See Shumpert, J. D., and Butler, C. M.,Penetration through slots in conducting cylinders-Part 1: TE case, IEEETrans. Antennas and Propagation, vol. 46, pp. 1612-1621, 1998; Shumpert,J. D., and Butler, C. M., Penetration through slots in conductingcylinders-Part 2: TM case, IEEE Trans. Antennas and Propagation, vol.46, pp. 1622-1628, 1998; Park, J. K., and Eom, H. J., Radiation frommultiple circumferential slots on a conducting circular cylinder, IEEETrans. Antennas and Propagation, vol. 47, pp. 287-292, 1999. Thesepapers present methods for modeling the EM field. However, the conceptand physical setup in communications applications is different from thatinvolved with logging applications. A key difference being the frequencyrange of operation: logging instrument antennas generally operate in EMdiffusion regime while communication antennas operate in propagationregime, where dimensions of antennas and slots are comparable towavelength.

It is desired to implement a technique to produce a pure transversemagnetic dipole EM field for subsurface formation measurements. Stillfurther, it is desired to implement a shield apparatus that can be usedin conjunction with saddle, tilted coils or multi-layered tri-axialcoils to produce such a field.

2. SUMMARY OF THE INVENTION

A shield apparatus adapted for use in conjunction with a logginginstrument provides predetermined attenuation of one or moreelectromagnetic energy field components as the field interacts with theshield.

One aspect of the invention is an apparatus for use with an elongatedsupport that is adapted for disposal within a borehole, the supporthaving a longitudinal axis. The apparatus comprises a body adapted toform a cylindrical surface; the body being adapted for mounting on thesupport; and the body having at least one slot formed therein such thatthe slot is perpendicular to the longitudinal axis when the body ismounted on the support; wherein the body provides predeterminedattenuation of an electromagnetic field component as the field interactswith the body.

Another aspect of the invention is an apparatus for use with anelongated support that is adapted for disposal within a borehole, thesupport having a longitudinal axis. The apparatus comprises a flexiblestrip adapted to surround the support, the strip being formed of anon-conductive material; and at least one conductive element disposed onthe strip such that the element is perpendicular to the longitudinalaxis when the strip surrounds the support; wherein the strip providespredetermined attenuation of an electromagnetic field component as thefield interacts with the strip.

Another aspect of the invention is a system for measuring a property ofa subsurface formation. The system comprises an elongated support havinga longitudinal axis, the support being adapted for disposal within asubsurface borehole traversing the formation; a source or sensor ismounted on the support; a shield is mounted on the support to cover thesource or sensor; and the shield has at least one slot formed therein,the slot being perpendicular to the longitudinal axis of the support;wherein the shield provides predetermined attenuation of anelectromagnetic field component as the field interacts with the shield.

Another aspect of the invention is a system for measuring a property ofa subsurface formation. The system comprises an elongated support havinga longitudinal axis, the support being adapted for disposal within asubsurface borehole traversing the formation; a source or sensor ismounted on the support; a flexible strip is mounted on the support tocover the source or sensor; and the strip has at least one conductiveelement disposed therein, the element being perpendicular to thelongitudinal axis of the support; wherein the strip providespredetermined attenuation of an electromagnetic field component as thefield interacts with the strip.

Another aspect of the invention is a method for shielding a source orsensor disposed on an elongated support having a longitudinal axis andadapted for disposal within a borehole. The method comprises mounting abody adapted to form a cylindrical surface on the support to cover thesource or sensor, the body having at least one slot formed therein suchthat the slot is perpendicular to the longitudinal axis, wherein thebody provides predetermined attenuation of an electromagnetic fieldcomponent as the field interacts with the body.

Another aspect of the invention is a method for shielding a source orsensor disposed on an elongated support having a longitudinal axis andadapted for disposal within a borehole. The method comprises mounting aflexible strip on the support to cover the source or sensor, the striphaving at least one conductive element disposed therein such that theelement is perpendicular to the longitudinal axis, wherein the stripprovides predetermined attenuation of an electromagnetic field componentas the field interacts with the strip

3. BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects and advantages of the invention will become apparent uponreading the following detailed description and upon reference to thedrawings in which:

FIG. 1a is a schematic diagram of a conventional cylindrical shield withaxial slots. Directed arrows are representative of the interactionbetween the shield and the electric field components of incidentelectromagnetic energy.

FIG. 1b is a schematic diagram of a conventional cylindrical shield withaxial slots. Directed arrows are representative of the interactionbetween the shield and the magnetic field components of incidentelectromagnetic energy.

FIG. 2 is a schematic diagram of a coil wound at an angle θ to thelongitudinal axis of the instrument. Also depicted is a view of thetilted coil as projected onto a two-dimensional surface.

FIG. 3 is a schematic diagram of a sloped slot pattern superimposed ontoa tilted coil and projected onto a two-dimensional surface. The slotsare maintained perpendicular to is the coil winding(s).

FIG. 4 is a schematic diagram of a sloped slot pattern superimposed ontoa non-tilted (axial) coil and projected onto a two-dimensional surface.

FIG. 5 is a schematic diagram of the sloped slot pattern of FIG. 4 withthe slots maintained centered over the coil winding(s).

FIG. 6 is a perspective view of a cylindrical shield in accord with theinvention.

FIG. 7 is a schematic diagram of a cylindrical shield in accord with theinvention. Dashed arrows represent the axial magnetic dipole andtransverse magnetic dipole components associated with the slot patternof the shield.

FIG. 8 is a schematic diagram of a shield composed of a strip in accordwith the invention. The strip is shown projected onto a two-dimensionalsurface.

FIG. 9 is a schematic diagram representative of a set of tilted magneticmoments oriented about a longitudinal axis.

FIG. 10 is an unwrapped view of a shield composed of a strip containingmultiple conductive elements in accord with the invention.

FIG. 11 is a diagram of the shield of FIG. 10 superimposed over thewindings of a tilted coil in accord with the invention.

FIG. 12 is a partial view of a shield illustrating a plurality ofhorizontal slots displaced along the planar surface in accord with theinvention.

FIG. 13 is a schematic diagram of a shield disposed on a support inalignment with and covering an antenna mounted on the support in accordwith the invention.

FIGS. 14a-14 c illustrate antenna configurations employing a transversecoil configuration using one or more saddle coils in accord with theinvention.

FIG. 15 is a partial view of a shield illustrating an axial slot formedbetween two rows of horizontal slots along the planar surface in accordwith the invention.

FIG. 16 is a partial view of a shield illustrating a plurality ofstaggered horizontal slots displaced along the planar surface in accordwith the invention.

FIG. 17 is a schematic diagram of a strip shield disposed on a supportin accord with the invention.

4. DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

As discussed above, conventional shields used in logging instrumentsuniversally have slots that arc aligned along the longitudinal axis ofthe instrument. The orientation of the slots is perpendicular to theelectric field generated by the source within or the field that is to bedetected by the sensor. If the incident field has an unwanted componentof the electric field that lies along the slot, then currents will flowin the metal to cancel that field and only the normal component willremain. For conventional induction or propagation instruments, thedesired electric field is azimuthal, and longitudinal slots allow thatfield to pass. If the coil was wound at an angle θ to the axis of theinstrument, then the desired electric field is no longer azimuthal, butrather has both azimuthal and longitudinal components that vary as afunction of the azimuthal position.

FIG. 2 illustrates a coil 14 wound at an angle θ to the longitudinalaxis (represented by dashed lines) of the instrument and having radiusa. Projecting the coil 14 onto a two-dimensional surface as shown, theheight of the coil 14 is described by a sinusoidal function of theazimuthal angle around the instrument φ:

ƒ(φ)=a tan θ cos φ.  (1)

sections. It will be understood by those skilled in the art that theshield 10 may be configured with various combinations of axial andtransverse slots, e.g., the shield 10 may comprise a sequence oftransverse slots with a plurality of axial slots (not shown).

FIG. 16 shows another shield 10 embodiment of the invention. Themechanical robustness of the shield 10 may be increased by the use ofstaggered transverse slots 12′. This shield 10 design provides otheradvantages, including allowing the use of 90° saddle coil antennas withseparate excitation of x and y directed TMDs (not shown). The strip 20shield described above may also be configured with transverse conductiveelements 22 to provide the desired EM attenuation (not shown). It willbe apparent to those skilled in the art, that various transverse-slotconfigurations may be implemented with the shields 10 of the inventionin conjunction with saddle, tilted coils or multi-layered tri-axialcoils. For example, the length of the transverse slots 12′ may shortenedand axial slots 34 may be interspersed in between the transverse slots(not shown). Another shield 10 implementation may include a crossed slotconfiguration (not shown).

Another embodiment of the invention involves a process for shielding asource or sensor disposed on a support that is adapted for disposalwithin a borehole. The process entails mounting a shield 10 of theinvention on the support to cover the source or sensor. The shield 10having at least one slot 12′ formed therein such that the slot 12′ isorthogonal to the longitudinal axis of the support. The shield 10 thusprovides predetermined attenuation of EM field components as the fieldinteracts with the shield.

Another embodiment of the invention involves a process for shielding asource or sensor disposed on a support that is adapted for disposalwithin a borehole. The process entails mounting a flexible strip 20 ofthe invention on the support 32 to cover the source or sensor 30. Thestrip 20 having at least one conductive clement 22 disposed therein suchthat the element 22 is orthogonal to the longitudinal axis of thesupport. as shown in FIG. 17. The strip 20 thus provides predeterminedattenuation of EM field components as the field interacts with thestrip.

Another embodiment of the invention involves the use of multiple tiltedcoils disposed at different angles so that the sum of their magneticmoments is in the transverse plane. A shield with transverse slots isthen placed over the antennas where the total electric field is vertical(not shown). A shield of the invention with a combination of verticaland

4.1 Sloped Slot Pattern

A shield to let pass the desired EM field components, and attenuate theundesired ones, should have at least one sloped slot that is sloped atan angle θ with respect to the instrument axis. A sloped slot patternfor a tilted coil 14, projected onto a two-dimensional surface, is shownin FIG. 3. The slots 12′ are perpendicular to the coil 14 at theintersection of the slot 12′ and coil 14. This allows the electric fieldcomponent that is parallel to the coil 14 to pass through the shieldwith minimal attenuation. This electric field will have azimuthal andaxial components, but no radial component. The slope of the slot 12′ isgiven by

1/(a tan θ sin φ).  (2)

Alternatively, one can represent the slots 12′ as a collection of pointmagnetic dipoles on the surface of a conducting cylinder (not shown).The location of each magnetic dipole is given by Equation (1), and theirorientation is given by Equation (2). Each individual magnetic dipolehas an axial component and an azimuthal component.

While the above discussion has assumed that the antenna under the shieldis tilted at an angle θ with respect to the instrument axis, the shields10 can also be used with an axial coil 14. With this configuration, theaxis of the coil magnetic dipole can be selectively rotated. FIG. 4illustrates a sloped slot 12′ pattern superimposed onto an axial coil 14and projected onto a two-dimensional surface.

As shown in FIG. 4, the slots 12′ are no longer perpendicular to thecoil 14 windings. This may affect the relative strength of the TMDcomponent to the axial magnetic dipole component. One approach tominimize these effects would be to maintain the slots 12′ centered overthe antenna, as shown in FIG. 5. FIG. 5 also shows a sloped slot 12′pattern superimposed onto an axial coil 14 and projected onto atwo-dimensional surface. Although the coil 14 in FIG. 4 is showncomprising multiple windings, it will be understood by those skilled inthe art that the shields of the invention are effective with coils 14composed of one or more windings.

While FIGS. 3-5 show straight slots 12′, in general the slots 12′ may becurved in order to maintain the direction of the slots 12′ perpendicularto the directions of the winding(s) or to keep them perpendicular to thedesired direction of the electric field that is to pass through theshield without attenuation. By surrounding an axial coil 14 with ashield 10, only the component of the electric field perpendicular to theslot 12′ will pass through without significant attenuation; thecomponents parallel to the slot 12′ will be significantly attenuated.The electric field that passes through the slots 12′ is in the directionthat would result from a true tilted antenna. Basically, the shield 10functions as a polarizer that passes components of the EM fieldcorresponding to a magnetic dipole oriented at an angle relative to theinstrument axis.

FIG. 6 shows an embodiment of a shield 10 configured as a hollow body 16with apertures 18 at both ends. The shield 10 is formed of a conductivematerial, typically metal. The ends 18 are adapted for connection to theinstrument using mating threads, fasteners, or other suitable meansknown in the art. Typical logging instruments consist of an elongatedmetallic pipe or mandrel as a central support means upon which sensors,electronics, and other instrumentation are mounted. It will beunderstood that other support means, such as coiled tubing ornon-metallic sondes, may be used to implement the invention, as theprecise type of support means is immaterial here. The hollow body 16 maybe open-ended or closed-ended. The body 16 is generally formed in theshape of a hollow cylinder. A right circular cylinder is preferable,although other shapes, such as an elliptical cylinder may be employed orvarious modifications to the cylindrical shape can be made to facilitatevarious other measurements. Preferably, a shield 10 will independentlysurround each coil on the instrument although a single longer shield mayalso surround multiple coils (not shown).

The EM radiation pattern around a logging instrument may be affected bythe instrument itself, so optimum shield 10 operation may require finetuning the exact slot 12′ pattern. Modeling shows that boreholeeccentricity can have a large deleterious effect on a measurement usingTMDs. Eccentered TMDs can couple directly into TM borehole modes via theTM mode's azimuthal magnetic field (Bφ). Since a tilted coil 14 can berepresented as a vector sum of an axial magnetic dipole and a transversemagnetic dipole, it will also be susceptible to large eccentricityeffects. However, the disclosed shield 10 configurations will providesome immunity to the TM mode, so the eccentricity effects may be reducedin severity. FIG. 7 shows the axial magnetic dipole component B_(A) andthe transverse magnetic dipole component B_(T) associated with each slot12′.

The shields 10 of the invention may be modified or combined to alter theeffects of incident EM energy. Multiple shields 10 may be overlaidcoaxially around an antenna. Combinations of sloped and axial slots ofvarying length, width, thickness, orientation, symmetry, density, orspacing may be formed on a shield 10. The sloped slots 12′ may haveequal or varied slope angles. The slots 12′ may be partially or entirelyfilled with some sort of lossy (i.e., conductive) material. A conductiveelement, such as a metallic strap or wire, may be connected between thesides of a slot 12′ to partially short out the slot 12′.

A shield 10 may also be formed comprising two halves or several sectionsconfigured to form a cylinder when combined (not shown). Such aconfiguration may further comprise one section or one half of the shield10 being electrically isolated from the other half or other sections.The spacing between the antenna and its support means or the spacingbetween the antenna and the shield 10 may also be varied. It will beappreciated by those skilled in the art having the benefit of thisdisclosure that other modifications may be employed to increase theefficiency of the shield 10.

4.2 Strip Shield

FIG. 8 illustrates another shield embodiment of the invention. A shieldmay be implemented in the form of a strip 20, also referred to as a flexcircuit. Flex circuit technology is similar to that used in conventionalmulti-layer printed circuit board where each layer may consist ofconductive regions on a resistive substrate. Connections can be madethrough the layers to points on other layers or to the outside. Thedifference with a flex circuit is that the substrate material isflexible and so after construction, the entire strip can be bent. Thestrip 20 is shown projected onto a two-dimensional surface for clarityof illustration. An effective strip 20 may be formed of any suitablenon-conductive material that can be adapted to coaxially surround theantenna. The strip 20 is preferably flexible, but it may also be formedof a rigid material. The strip 20 contains at least one conductiveelement 22, preferably a multitude of elements 22. The conductiveelements 22 may be formed of fine strips of copper or other suitableconductive materials.

As described above, a shield incorporating sloped slots may be used torotate the magnetic moment of an antenna. Thus, the conductive elements22 are disposed in the strip 20 such that each element 22 is sloped atan angle with respect to the instrument axis when the strip is mountedon the instrument to surround the antenna. Since the strip 20 isnon-conductive (unlike the shield embodiments described above), theelements 22 must also be configured to form a loop around the antennawhen the strip surrounds the antenna. The loop provides the path inwhich currents can flow around the antenna in order to rotate the axisof the magnetic dipole. The strip 20 provides selective attenuation ofthe EM energy emitted or received by an antenna when a complete loop isformed around the antenna by the conductive element 22.

A switchable connection is provided in the strip 20 to selectively openor close the loops formed by the conductive elements 22, as illustratedin FIG. 8. This connection may be a series of connections or only oneconnection. The connection(s) may also be located at any suitable pointin the circuit. When the connection is closed, the element 22 acts torotate the antenna's magnetic dipole. When it is open, it has no effect.One form of a switchable connection utilizes a MosFET switch to open orclose the current path around the antenna. Other suitable means may beutilized to form the switchable connection(s) as known in the art. Thestrip 20 may also comprise additional switching means (not shown) toprovide an electrical short with the support member if desired.

The strip 20 may be modified or combined to alter the effects ofincident EM energy. Multiple layers of conductive elements 22 havingdifferent directions of magnetic dipole moments may also be disposed onthe strip 20. This would allow the use of a single axial coil 14 as atransmitter or receiver and by closing the switchable connection(s) onthe strip 20, different rotations of the magnetic moment could beachieved. Alternatively, multiple strips 20 could be overlaid coaxiallyto surround an antenna.

4.3 Directional Measurements

By altering the direction of the magnetic dipole, an antenna can be usedto make formation measurements at multiple orientations. This sectiondescribes a method for winding and shielding an antenna structure toproduce a set of TMDs.

By superimposing or overlaying three coils around a support means andwrapping the coils with one or more strips 20, a tri-axial dipole setmay be produced. FIG. 9 illustrates a set of magnetic moments directedalong three orthogonal directions at an equal angle to the longitudinalaxis of the instrument. With this configuration, the three antennas andtheir corresponding strip(s) 20 can be turned on or off independently.This allows for any one antenna and polarizer pair to be engaged, whilethe other two sets are disengaged.

The construction of an antenna and polarizer strip 20 for the simplestcase (which would be just one coil and its corresponding polarizer) willnow be described. The coil may be wound around a support (such as aninsulated mandrel) from any suitable conductive wire as known in theart. Referring to FIG. 2, to produce a magnetic dipole at some angle Φbetween 0° and 90°, the location of the center of the thread shouldfollow

Z(φ)=−a tan Φ cos φ+pφ,  (4)

where a is the radius of the support means, φ is the azimuthal angle,and p is the pitch. The wire is preferably wound closely packed so thatthe thread depth and width are on the order of the wire diameter d anda>>p≧d where d is the wire diameter.

The polarizer strip 20 may be constructed so that the conductiveelements 22 are everywhere perpendicular to the current in the coilwindings. FIG. 10 shows an embodiment of a strip 20 containingconductive elements 22. The conductive elements may be embedded, glued,or affixed to the strip in any suitable manner as known in the art. Thefunctional form ƒ(φ′) of these conductive elements 22 is derived by$\begin{matrix}{{{f\left( \varphi^{\prime} \right)} = {\int{\frac{- 1}{\frac{z}{\varphi}}{\varphi^{\prime}}}}},} & (5)\end{matrix}$

where $\begin{matrix}{{\frac{z}{\varphi} = {{- a}\quad \tan \quad {\theta sin}\quad \varphi^{\prime}}},} & (6)\end{matrix}$

evaluated at φ=φ′. Therefore, $\begin{matrix}{{{f(\varphi)} = {{\int\frac{1}{\beta \quad \sin \quad \varphi}} = {\frac{1}{2\beta}{\ln \left( \frac{1 + {\cos \quad \varphi}}{1 - {\cos \quad \varphi}} \right)}}}},} & (7)\end{matrix}$

where β=a tan Φ.

In addition to providing selective attenuation of EM energy components,the polarizer strip 20 acts as a Faraday shield to reduce capacitivecoupling between antennas, without attenuating the desired components ofthe magnetic field. The behavior as a Farady shield is comparable to thebehavior of conventional shields used on present generation induction orpropagation instruments. FIG. 11 shows the strip 20 of FIG. 10superimposed over the windings 24 of a tilted antenna. As shown in FIG.11, the conductive elements 22 are everywhere perpendicular to the coilwindings. Although FIG. 11 shows the superposition of a strip 20 over acoil 14, the same pattern applies to the superposition of a cylindricalshield 10 with sloped slots 12′ over a coil 14. The simplified antennaand strip 20 described above can be overlaid to create a set of basismagnetic dipoles. These can be used to construct an antenna structurethat provides selective three-dimensional measurement capability.

4.4 Transverse Slots

A transverse magnetic dipole antenna is a key building block fortri-axial and directional measurements. To generate a pure transversemagnetic dipole EM field, a shield configured with a transverse slot ispreferred. FIG. 12 shows a shield 10 embodiment of the invention. Aplurality of horizontal slots 12′ are spaced along the body of theshield 10. The slots 12′ are preferably parallel to one another. Similarto the shield embodiments discussed above, the optimal shield 10 for apure transverse magnetic dipole EM field should have slots that areperpendicular to coil excitation.

FIG. 13 shows a shield 10 covering an antenna 30 disposed on a supportmember 32. The antenna 30 comprises a transverse coil configurationusing one or more saddle coils. Turning to FIG. 14a, an antenna 30 isillustrated having segmented coils 602 and 604. These segmented coilstogether produce a magnetic dipole 608 that extends radially from thesupport (represented by the dashed line). As is generally illustrated,the segmented coils 602, 604 are formed to extend about thecircumference of the support. We refer to this as a saddle coil, becauseits shape resembles that of a saddle. It consists of a circular arc atthe top and bottom of the coil connected by a longitudinal segment.Often we will have a pair of these coils disposed onazimuthally,opposite sides of the support member of the instrument. Thecoil segments 602, 604 may be connected in series to insure equalcurrent parameters, or they may be connected in parallel if desired.Alternatively, the segmented coils 602, 604 may be independentlydisposed on the support and energized to produce the magnetic dipole.

Turning to FIG. 14b, which is an axial view of the instrument, anotherantenna 30 embodiment includes a second set of half-coils 622, 624 thatorient and receive current so as to produce a magnetic dipole 628 thatalso extends radially from the support on which the half-coils aremounted. Half-coils 602 and 604 are overlaid to surround half-coils 622and 624. The half-coils 622, 624 are disposed on the support to producethe magnetic dipole 628 so that dipole 628 is rotated azimuthally withrespect to the magnetic dipole 608. The design of half-coils 622 and 624is similar to the design of half-coils 602 and 604, however they arerotated azimuthally with respect to the previous set. FIG. 14c furtherillustrates the orientation of these magnetic dipoles 608, 628. Thesemagnetic dipoles 608 and 628, disposed within the borehole 630, arecontrollable so that the measurement sensitivity may be directed axiallyfrom the support at any azimuth angle.

With the use of a saddle coil antenna 30, since the excitation currentis primarily longitudinal (z-directed), the corresponding slots 12′ areazimuthal and the shield body is centered over the center of the antenna30, as shown in FIG. 13. By varying the number, dimensions, and/ordisplacement of the slots 12′ on the shield 10 surface, the attenuationof interacting EM field components is altered. Attenuation of thesefield components may be reduced by using one or more axial (vertical)slots in combination with the transverse slot(s) 12′. FIG. 15 showsanother shield 10 embodiment of the invention. An axial slot 34 isdisposed between two sequences of transverse slots. 12′ such that whenthe shield 10 is mounted, the slot 34 is parallel to the support axis.Axial slots 34 could also be disposed over the upper and lower parts ofthe saddle coils where the coils are composed of azimuthal sections. Itwill be understood by those skilled in the art that the shield 10 may beconfigured with various combinations of axial and transverse slots,e.g., the shield 10 may comprise a sequence of transverse slots with aplurality of axial slots (not shown).

FIG. 16 shows another shield 10 embodiment of the invention. Themechanical robustness of the shield 10 may be increased by the use ofstaggered transverse slots 12′. This shield 10 design provides otheradvantages, including allowing the use of 90° saddle coil antennas withseparate excitation of x and y directed TMDs (not shown). The strip 20shield described above may also be configured with transverse conductiveelements 22 to provide the desired EM attenuation (not shown). It willbe apparent to those skilled in the art, that various transverse-slotconfigurations may be implemented with the shields 10 of the inventionin conjunction with saddle, tilted coils or multi-layered tri-axialcoils. For example, the length of the transverse slots 12′ may shortenedand axial slots 34 may be interspersed in between the transverse slots(not shown). Another shield 10 implementation may include a crossed slotconfiguration (not shown).

Another embodiment of the invention involves a process for shielding asource or sensor disposed on a support that is adapted for disposalwithin a borehole. The process entails mounting a shield 10 of theinvention on the support to cover the source or sensor. The shield 10having at least one slot 12′ formed therein such that the slot 12′ isorthogonal to the longitudinal axis of the support. The shield 10 thusprovides predetermined attenuation of EM field components as the fieldinteracts with the shield.

Another embodiment of the invention involves a process for shielding asource or sensor disposed on a support that is adapted for disposalwithin a borehole. The process entails mounting a flexible strip 20 ofthe invention on the support to cover the source or sensor. The strip 20having at least one conductive element 22 disposed therein such that theelement 22 is orthogonal to the longitudinal axis of the support. Thestrip 20 thus provides predetermined attenuation of EM field componentsas the field interacts with the strip.

Another embodiment of the invention involves the use of multiple tiltedcoils disposed at different angles so that the sum of their magneticmoments is in the transverse plane. A shield with transverse slots isthen placed over the antennas where the total electric field is vertical(not shown). A shield of the invention with a combination of verticaland transverse slots (such as shown in FIG. 15) could be used with a setof tilted coils to produce a magnetic moment with arbitrary direction.

While the methods and apparatus of this invention have been described asspecific embodiments, it will be apparent to those skilled in the artthat other embodiments of the invention can be readily devised which donot depart from the concept and scope of the invention as disclosedherein. All such similar variations apparent to those skilled in the artare deemed to be within the scope of the invention as defined by theappended claims.

What is claimed is:
 1. An apparatus for use with an elongated supporthaving at least one antenna adapted to transmit or receiveelectromagnetic energy mounted thereon, said antenna adapted to producea substantially transverse magnetic moment when activated to transmitelectromagnetic energy, said support having a longitudinal axis andadapted for disposal within a borehole, comprising: a flexible stripadapted to surround said support in alignment with the at least oneantenna, said strip being formed of a non-conductive material; and atleast one conductive element disposed on said strip such that saidelement is perpendicular to the longitudinal support axis when saidstrip surrounds said support; wherein said strip attenuates interactingelectromagnetic field components to facilitate a substantiallytransverse magnetic dipole field near the at least one antenna.
 2. Theapparatus of claim 1, wherein said at least one conductive element isdisposed on said strip to form an open loop around said support whensaid strip surrounds said support.
 3. The apparatus of claim 2, furthercomprising switching means connected to said at least one conductiveelement, said switching means being operative to provide selectiveclosure of said open loop to form a closed loop.
 4. The apparatus ofclaim 1, wherein said strip comprises multiple conductive elementsdisposed therein such that each element is perpendicular to saidlongitudinal axis when said strip surrounds said support.
 5. A systemfor measuring a property of a subsurface formation, comprising: anelongated support having a longitudinal axis, said support being adaptedfor disposal within a subsurface borehole traversing said formation; asource or sensor mounted on said support; said source or sensorrespectively adapted to produce a substantially transverse magneticmoment when activated or to detect a signal associated with asubstantially transverse magnetic moment; a flexible strip mounted onsaid support to cover said source or sensor; and said strip having atleast one conductive element disposed therein, said element beingperpendicular to said longitudinal axis of said support; wherein saidstrip attenuates interacting electromagnetic field components tofacilitate a substantially transverse magnetic dipole field near thesource or sensor.
 6. The system of claim 5, wherein said at least oneconductive element is disposed on said strip to form an open loop. 7.The system of claim 6, further comprising switching means connected tosaid at least one conductive element, said switching means beingoperative to provide selective closure of said open loop to form aclosed loop.
 8. The system of claim 5, wherein said source or sensorcomprises an antenna having a magnetic dipole moment and adapted totransmit and/or receive electromagnetic energy.
 9. The system of claim8, wherein said antenna is disposed on said support such that saidmagnetic dipole moment is tilted or perpendicular with respect to saidlongitudinal axis of said support.
 10. The system of claim 9, whereinsaid strip is mounted on said support such that said at least oneconductive element is positioned over said antenna.
 11. The system ofclaim 8, wherein said antenna comprises a saddle coil.
 12. The system ofclaim 5, wherein said strip comprises multiple conductive elementsdisposed therein such that each element is perpendicular to saidlongitudinal axis of said support.
 13. The system of claim 12, whereinsaid multiple conductive elements are asymmetrically spaced on saidstrip.
 14. A method for shielding a source or sensor, respectivelyadapted to produce a substantially transverse magnetic moment or todetect a signal associated with a substantially transverse magneticmoment, disposed on an elongated support having a longitudinal axis andadapted for disposal within a borehole, comprising mounting a flexiblestrip on said support to cover said source or sensor, said strip havingat least one conductive element disposed therein such that said elementis perpendicular to said longitudinal axis, wherein said stripattenuates interacting electromagnetic field components to facilitate asubstantially transverse magnetic dipole field near the source orsensor.
 15. The method of claim 14, wherein said source or sensorcomprises an antenna having a magnetic dipole moment and adapted totransmit and/or receive electromagnetic energy.
 16. The method of claim15, wherein said antenna is disposed on said support such that saidmagnetic dipole moment is tilted or perpendicular with respect to saidlongitudinal axis of said support.
 17. The method of claim 16, whereinsaid strip is mounted on said support such that said at least oneconductive element is positioned over said antenna.
 18. The method ofclaim 17, wherein said antenna comprises a saddle coil.